The present invention relates to the manufacture of tamper-proof, fluid-tight, captive plastic closure of the type comprising a cap portion having a flat, bulged or cup-shaped bottom, an outer skirt provided with a guaranty strip adapted to be at least partially torn off, fitted between the cap edge and a retaining ring, and a ribber internal contour adapted to co-act with a matching ribbed external contour of the neck of the container to be stopped by means of this closure.
The manufacture of closures of this type requires the use of two-section moulds having lateral shells on either side of the vertical joint plane. In fact, after the removal of the upper portion of the mould having a contour corresponding to the cap contour, the side faces of this cap must be uncovered to permit the release of the skirt with its guaranty strip bounded by two circular grooves formed externally and corresponding to a substantial reduction in the cross-sectional area of the closure material. Now the manufacture of these mould shells requires a high standard of precision. Therefore, they are relatively expensive and nevertheless burrs of plastic material are likely to develop even under normal service conditions. Moreover, the use of such shells increases the risk of failures during the mould operation, for they are extremely fragiles and their wear involves and expensive replacement thereof at regular time intervals. Furthermore, in a given injection press type, the over-all dimensions of the mould shells limits detrimentally the number of moulding cavities or impressions that can be formed in one mould.
Now the present invention provides a modified mould structure and arrangement permitting of eliminating the above-defined mould shells while forming on the closure plain lateral surfaces preventing any tampering with the guaranty strip, with substantial production savings and an appreciable improvement in the manufacturing efficiency.